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Fluoropolymer heat exchangers
FLUOROPOLYMER HEAT EXCHANGERS by S. R. Wharry, Jr. Ametek/Haveg Div.. Wilmington. Del
Fluoropolymer heat exchangers are the only polymeric heat exchangers to offer the temperature capabilities and corrosion resistance necessary to heat and cool acid- and caustic-based plating bath s. They offer economical solutions to the problems of maintaining precise temperature control of baths for optimum product quality.
INTRODUCTION Polymeric heat exchanger usage has become Widespread throughout today's industry for heat-transfer applications in highly corrosive situations. The principal reason that polymeric heat exchangers are used instead of conventional metal heat exchangers is that all metals are subject to corrosion in chemical media . Althou gh the corrosion rate of certain metals may be acceptable in the chemical species utilized. they all suffer corrosion. and when aggressive chemical media are used. the rate of corrosion becomes prohibitive to economical operation. There are many types of thermoplastic and other polymeric materials available in heat exchangers (polypropylene and polyethylene have been used for some very low-performance applications), but only a few polymers exhibit the chemical and temperature capabilities that make them suitable for widespread heat-exchanger application. In general, the polymers that exhibit these properties are the fluorocarbon family of resins . They are available from several different resin manufacturers and each exhibits a general range of acceptable usage.
FLUOROPOLYMER STRUCTURE AND PROPERTIeS The wide acceptance of fluoropolymer heat exchangers can primarily be attributed to the unique properties of the fluorocarbon polymers. Fluoropolymers are corrosion res istant to almost all chemicals. with the key to this corrosion lying in the chemical structure. Polytetrafluoroethylene (PTFE). perfluoroalkoxy (PFA). and fluorinated ethylene propylene (FEP) fluoropolymers are fully fluorinated polymers; that is. each branch terminates with a fluorine atom (see box) . Polyvinylidene fluoride (PVDF). ethylene tetrafluorethylene (ETFE). and ethylene-ehlorotrifluoroethylene (ECfFE) are only partially fluorinated (some branches do not end with a fluorine atom) . This fully fluorinated structure provides a polymer that is both chemically inert and thermally stable to high temperarures. Partially fluorinated polymers sacrifice some chemical and thermal resistance 10 enhance their mechanical properties at room temperature. This results in the higher ambient temperature tensile strength and modulus shown in Table I. As can clearly be seen. however. the upper operating limits of the PVDF, ETFE, and ECTFE are severely restricted and FEP and PFA have higher temperature limits. This is important in heating applications when thermal margins of safety can be extremely important. One can also readily see that the fully fluorinated structure of FEP and PFA (fable II) offers superior chemical resistance compared with the other part ially fluorinated polymers. Thi s is especially true in the area of strong alkalies, polar solvents. and strong acids. Two fluoropolymer family members. EFTE and ECfFE. have to date found more applications in structural and lining applications and not in heat-exchanger equipment.
THERMAL PROPERTIES Fluoropolymers are not good heat conductors. Shown in Table nr is a tabulation of thermal conductivity values comparing the various fluoropolymers directly with those of
769
Table I. Properties of Fluorinated Polymers
PTFE
Type
Specific weight Melting point
glee
Tensilestrength
mPa psi mPa psi
2.17 327 621 30 4,350 10 1,450 500
'C
'F
Yield stress Elongation Elasticmodulus
%
mPA psi XIlY
600
87 60 160
Shore D hardness Impact strength (1200)
ft-Ib/in. 2 Heat Distortion Temperature 66 psi (0.46 mPA) Linearexpansion coefficient Thermal conductivity
2.15 270 518 20 2,900 12 1,740 350 500 72 57 200
PFA 2.15 305 581 31 4,500 15 2,250 300 700 101 62
PVDF
ETFE
1.78 178 350 54 7,830 46 6,670
1.75 275 527 24 3,480 24 3,480 500 1,500 217 75
1.7 240 464 31 4,500 31 4,500 300 1,650 240 75
ISO 2,400 348 79
ECTFE
JIm 3 "C
'F
1O-5K
BTU/hr/ft'
4
121 250
ISO
70
73 164
148 300
104 219
115 240
13
13
12
12
12
12
0.20 1.4
0.23 1.6
ft2 -"Ffm Upperservice temperature
FEP
"F
400 205
500 260
"C
0.22 1.5 500 260
0.19 1.3 300
ISO
0.23 1.6 280 140
0.15 1.0 340 170
EcrFE; ethylene-chlorotrifluoroethylene. ETFE, ethylene tetrafluoroethylene: FEP. fluorinated ethylene propylene. PFA. perfluoroalkoxy, PTFE. polytetrafluoroelhylene, PVDF, polyvinyhdene fluonde,
metals. In light of these relatively poor thermal properties, why then does a fluoropolymer work as a practical heat exchanger? When dealing with a heat-transfer situation, one first determines the amount of heat necessary to be transferred. It is then simply a direct function of the heat-transfer coefficient or the capability of the heat exchanger, the area available for heat exchange, and the temperature driving force used to promote that heat transfer.
Q
=
UA~TL
(1)
Here, there is a log mean temperature difference, because in the dynamic situations occurring during tank heatup or cooling, there is a constantly changing system, not a constant temperature. The log mean temperature is defined by the following equation:
~TL
(2)
Note that it is not a simple temperature difference with which one must deal. The main consideration in this article, however, is calculation of the heat-transfer coefficient, as it develops from the relatively low thermal conductivity.
U (3)
770
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~
Table II. Chemical Resistance of Fluoropolymer Resins
PTFE
FEP
PFA
PVDF
Media
20t:
6Ot:
9Ot:
20t:
6Ot:
9Ot:
20t:
6Ot:
9Ot:
20t:
6Ot:
9Ot:
H 2S04 (15%) H 2S04 (98%)
A A A A A A A A A A A A A
A A A A A A A A A A A A A
A A A A A A A A A A A A A
A A A A A A A A A A A A A
A A A A A A A A A A A A A
A A A A A A A A A A A A A
A A A A A A A A A A A A A
A A A A A A A A A A A A A
A A A A A A A A A A A A A
A A A A A A A B B A B A A
A A A A A A A B B A U A A
A A A A A A B U U A U B B
aci
(36%)
HNO, (50%) Nitric (20%)/HF (5%)
HF (40%) Chromic (50%) NaOH (50%) NaOCI
0; (wet) Ketones, esters Aromatics Chlorinated solvents
A, acceptable, B, condlllonaIly acceptable, U, unsatisfactory. FEP, fluonnated ethylene propylene; PFA, perfluoroalkoxy, PTFE,polytetrafluorethylene; PVDF, polyvinylidene fluonde.
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nickel/ sulfuric anod izing). The modular custom design permils conformityto all)' vessel geo melly. The patented inieclion averrnolding ensures permanent attachment of the tubing to exchanger manifolds. Available in immef1)ion as wellas shell-and-tubetype design.
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Table III. Coefficients of Thermal Conductivity
tr- 'FUn.
Material
Thermal Conductivity W/M"K.
BTU/hr
Copper Graphite Nickel Tantalum Carbon steel Titanium Stainless steel Ni-Cr-Mo PFA
2.700 IBO 619 368 348 III 104 56 \.6 1.5
FEP
1.4
PVDF
\.3
PTFE
390 120 89 50 50 16
15 8 0.23 0.22 0.20 0.19
FEP. fluorinated ethylene propylene; PFA. pernuoeoalkoxy: PTFE. polytetrafluoroethylene; PVDF. polyvinyltdene fluoride.
In this simple definition of the heat-transfer coefficient (Eq. 3), the individual film coefficients are summed up; i.e., what happens to the fluid outside the tube in the bath (ho ) . conduction of heat through the wall (hw). transfer of heat between the fluid in the tubes to the wall (hi)' plus fouling factors. This summation yields the overall coefficient or U value, which is a measure of the ability of a heat exchanger to transfer heat. Taking the example shown here and applying typical data, film coefficients for inside the tubing (condensing steam 500 BTU/hr/ft 2 oP or greater) and in the surrounding bath (200 to 300 for agitated systems) can be inserted. Typical textbook fouling factors are 0.001. If small-diameter tubing is considered, typically \1i-in. diameter tubing with a 0.00125- in. wall (discussion of this typical value is later in the text), the conductivity calculates to a coefficient of 96 BTU/hr/ft 2oF. U
I 500
-
I 96
I 300
+- +-
57.8
+ 0.001 + 0.001
(4)
Plugging all of these values into the equation for the overall heat-transfer coefficient (Bq. 4), it is obvious that the most significant and controlling factor in this calculation is the wall of the tubing. The order of magnitude of this resistance is greater than that in the fluids on either side, and especially so for the fouling factors. For this reason, when sizing any fluoropolymer heat exchanger, fouling factors are not considered to be an important item. Since the order of magnitude is so small, compared with the controlling factor, it becomes mathematically insignificant. The nonstick character of fluoropolymers further enhances the logic that fouling factors are not important. This U value of approximately 60 is typical for steam heating in a plating bath with pure fluoropolymer tubing. The wall conductivity of a metal heat exchanger (based on thermal conductivity is approximately 75 times that of the fluoropolymer heat exchanger) results in an overall heat-transfer coefficient typically three times that of the tluoropolymer heat exchanger. If the mathematics as shown in Equation 5 are reviewed, it can be seen that the order of magnitude of the fouling factors is very significant for a metal heat exchanger, and, therefore, must always be considered when calculating necessary heat-transfer area.
U
= -
I
500
I
I
1500
300
+ -- + -
150
+ 0.001 +0.001
(5)
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Coating and Surface Treatment Systems for Metals -A Comprehensive Guide to Selection by j. Edwa rds 470 p ages
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Selectio n of the most appropriate coating or other surface treatment is addresse d in this comprehe nsive guide , Part r covers 76 industrially important coating types from acrylic polymers through zinc alloys. Part U prov ides an overview of the 19 most important coating treatment methods with emphasis on the implications for a particular product in terms of its substrate or shape . Part III offers a guide to coating characteristics.
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Approximately 4 yearsago, a fluoropolymer tubingcalled "Q" tubing was introduced. It effectively doubledthe thermal conductivity over that of all pure fluoropolymers available. If that wall conductivity number is doubledto approximately 200 BTU/hr/ft2°F, the wall is still the controIling factor, but now you have closed the gap considerably with metals (Eq, 6).
FLUOROPOLYMER STRUCTURES AND PROPERTIES PTFE-Polytetrafluoroethylene: MeltingPoint620·P(327·C) F P P F ServiceTemperature SOO°F (260°C) I I I I -C-C-C-CChemically inert Insoluble I I I I
F F F F
FEP·Fluorlnated ethylene propylene:
F F P F
I I I I
-C-C-C-C-
I I I I
MeltingPointS40°F (280°C) ServiceTemperature 400"F(20S·C) Chemically inert Insoluble
F F F CF3 PFA-Perfluoroalkoxy:
F F F F
I I I I
-C-C-C-C-
I I I I
F F F OC3F7 ETFE·Ethylene tetrafluoroethylene: F F H H
I I I I
-C-C-C-C-
I I I I
F F H H PVDF-Polyvlnylldene fluoride: F H F H
I I I I
-C-C-C-C-
I I I I
MeltingPointS80°F (301°C) ServiceTemperature SOO"P (260°C) Chemically inert Insoluble
MeltingPointS12·F(267°C) ServiceTemperature 300"F(I SO·C) Reacts-strong acids/bases Insoluble MeltingPoint320°F(160°C) ServiceTemperature 280°F(I40·C) Reacts-strong acidslbases Solubleinketones
P H P H
ECIFE-Ethylene-chlorotriOuoroethylene: F F H H MeltingPoint46S·F (240·C) I I I I ServiceTemperature 340·P(I70·C) - C- C- C- CGood-acidslbases I I I I Poor-organics & solvents P CI H H
n6
Table IV. Thermal Efficiency versus Tubing Diameter Tubing Diameter
Wall Conductance Coefficiau BTUlhr1tr- m«
(in .;
(mm)
Type
Kca/lhrM~= "C
0.100 0.100 0.125 0.125 0.175 0.175 0.250 0.250 0.3175 0.3175 0.375 0.375
2.54 2.54 3.18 3.18 4.45 4.45 6.35 6.35 8.06 8.06 9.53 9.53
FEP
586 1.170 468 928 336 668 234 468 185 366 156 322
Q
FEP Q
FEP Q
FEP Q
FEP Q
FEP Q
120 240 96
19<1 69 137 48 96 38 75 32 64
F£P. IIUOnllal@ eihylenepropylene; O. a Ituorpolymcrtu6mg
U
I
I
I
-+-+-+0.001 +0.001 500 200 300
84 (6)
The coefficient now typically ranges up to 120 BTU/hr/ft2'F against a typical metal heattransfer coefficient range of the order of 150 to 300 BTU/hr/ft2F. What one must do in a fluoropolymer heat exchanger to match the performance of a metal unit is. therefore. to specify more surface area; however. because of the generally smaller diameter of the tubing utilized in f1uoropolymer heat exchangers. the surface area available per running foot or per cubic foot of heat exchanger is significantly higher than that of the metals; therefore. the unit. while it contains more surface area. does not occupy a higher volume and does not detract from available bath area.
FLUOROPOLYMER HEAT EXCHANGERS Fluoropolymer heat exchangers are generally manufactured with small-diameter tubes. Functionally. it becomes very difficult to gain heat-transfer efficiency in tubing above 'h-in. diameter. The wall thickness required to provide structural strength of tubing larger than that reduces the thermal efficiency considerably. Table IV shows the various sizes of tubing available in units in both FEP and Q. The smaller diameters have significantly higher wall conductivity and, therefore. create a much more thermally efficient unit; however. the smaller diameter tubes are generally suitable only for clean fluids inside the tubes . One generally makes immersion units out of smaller diameter tubes where clean steam or cooling water flows in the tubes. Larger chemical process units are made out of the larger diameter tubes where dirty process fluids are introduced tubeside in the heat exchangers. Units of this latter type are also appropriate for external heaters/coolers for metal finishing baths. Because tubing is extruded from fluoropolymer resins. pressure capability is a strong function of temperature. The pressure capabilities of the various fluoropolymer resins are very much regulated by the temperature of the application as well as the upper temperature capabilities of the various polymers (Fig. 1). Most manufacturers of f1uoropolymer heat exchangers follow a similar pattern. Tubing is typically manufactured with what is called a 10% wall. In other words. the wall thickness is 10% of the diameter of the tubing. Some tubing can be made as low as 8% wall without serious effects on its pressure capability. With the use of the 10% rule for wall thickness. all tubing within this size range (less than 'h in.
m
'C
160
10
·IS
\
140
\
n
]I
149 .. ..177- - - "204
1103
9n
\
821
\ Internal P,..uure 1. Tubl.9
\. \
~
~
~ ~EP
40
~
Eatlrml 'rlssu'" 20
121
0" Tubtll9
~ ~
"PfA
~ i'-. """--
~ ...JA
21S
us
f
fEP
o so
100
ISO
200 2SO MIl .... TDl'EIlATUIlE 'f
JOO
3S0
coo
Fig. 1. Operating limits. diameter) has generally similar temperature/pressure capabilities. Differences in tubing capability created by the hoop stresses in this size range are relatively insignificant and considered to be nonexistent. COMMERCIAL USE This discussion will focus on what can be broadly characterized as the metal finishing industry. Technically, all establishments that apply coatings or surface treatments to a metal item are part of the metal finishing industry. Many additional operations also fit into this general category, such as mechanical polishing, painting, and the application of other coating materials. These areas seldom require heat exchange and, as such, the involvement is limited to the plating and similar chemical or electrochemical finishing operations where corrosive solutions and heating or cooling are involved. Some categorizations of the type of operations included are: • • • •
Decorative plating Hard chromium plating Precious metal plating Chloride , cyanide, fluoride, fluoborate, fluosilicate, pyrophosphate, sulfate, and sulfamate plating solutions for cadmium, chrom ium, copper, gold, lead-tin, nickel , platinum, rhodium, silver, tin, and zinc electroplating • Electroless copper and nickel plating solution s • Deionized water • Anodizing, brightening, clean ing, electrocleaning, electropolishing, etching, neutralizing, passivating, pickling, rinsing, stripping, and surface-activating solutions containing alkalis or acetic, chromic, hydrochloric, hydrofluoric, nitric, oxalic, phosphoric, or sulfuric acids Some of these applications are designed to provide decorative finishes to make the article more attractive . More importantly, they can provide funct ional finishes that impart specific
na
Fig. 2. Minicoi/.
Fig. 3. Supercoi/.
engineering properties such as surface hardness and resistance to wear. abrasion. or corrosion to the article being plated. In all of the above operations, precise control of bath temperature at an optimum condition is critical to obtaining a product of the highest quality. Baths must be heated from room temperature to provide the optimum temperature for processing and then maintained at that condition, either through additional intermittent heating or by cooling. especially in the case of electroplating where the electric energy that is input into the bath is translated into significant heat values. Fluoropolymer heat exchangers can provide heating as well as cooling in a single unit. Electric or flame heating requires a second unit for cooling. In today's metal finishing baths, heating and cooling with fluoropolymer heat exchangers can be accomplished in a number of ways. The first, and perhaps the simplest, technique is an immersion heater/cooler placed directly in the plating bath itself. Several types of these heat exchangers are available. The smallest of these units is a board- or frame-mounted unit available from several vendors (Fig. 2). They are most typically used in small plating baths, but through modular-type construction can provide significant area. They have the deficiency of being fabricated from '.!.I-in. or larger diameter tubing, which provides an inefficiency of operation because of the thicker walls. They also are of a single-tube design, requiring individual tube connections into manifolds for operation with the heating or fluid media. The single-tube design can also limit coolant flow. The second category of immersion coil is that typified by the supercoil (Fig. 3) in sizes ranging from approximately IOto 75 ft2 in heat-transfer area. These are produced in both FEP and Q tubing utilizing Vla- and 'h-in. diameter tubing. These types of coils utilize a process called honeycombing. which fuses the many small-diameter tubing ends into a single fluorocarbon sheath (which is provided with a mechanical connection to a standard NPT Teflon or steel piping connection). In this way. a client can connect the coil utilizing standard supply piping and is not required to manifold many small tubes individually. Larger coils are
779
Fig. 4. S/im/ine. also produced called slimline immersion coils (Fig. 4). These can provide up to 250 ft2 in a single coil and are held in a rigid configuration by external Teflon-covered steel rods. Of course, external tank heating is also a possibility and is utilized by many metal processors. This is simply utilization of a shell and tube unit with fluoropolymer tubes contained in a steel or noncorrosive shell for the heating/cooling media (Fig. 5). Detailed selection of materials can be made for the shell and sealing gaskets, depending upon the nature
Fig. 5. External bath heating.
780
of the media involved. In shell and tube units . it is advisable to use slightly larger diameter tubing , as the bath solution is being circulated through the tubing of the heat exchanger, and particulate contamination as developed in the bath can be circulated through the unit .
ADVANTAGES OF FLUOROPOLYMER HEAT EXCHANGERS Fluoropolymer heat exchangers are true heat exchangers. They are not heaters. They can be used for heating and cooling of corrosive bath environments. Follow ing are some of the advantages compared with some of the competitive heating techniques. • F1uoropolymer heat exchangers have near universal corrosion resistance. This results in long life, no plating bath contamination from corrosive products, and a smaller spares inventory because f1uoropolymers work in all solutions, which provides extreme versatility. You can use the same heat exchangers even though the bath composition is changed. • F1uoropolymer heat exchangers are electrically nonconductive. They are simple. easy to install, and electrically insulated pipe connections are not needed. You get improved plating efficiency because stray currents are not attracted to a nonconductive fluoropolymer heat exchanger. There is no shock hazard. Generally, operating costs are lower than with electric heat . There are no burned-out heaters, which need frequent replacement. • Units are safer to operate as there is no risk of tank ignition or damage to tank lining if solution level drops. This compares not only to an electric heater situation, but also to nonelectric heat sources, such as flame or carbon "hot sticks ." • F1uoropolymers resist fouling and plateout. So, surface passivation may be eliminated in certain solutions. • There is longer bath life. and less bath depletion as compared with direct steam or hot water injection. There is less waste for disposal or recovery. • Original heat transfer performance is maintained. It does not deteriorate with plateout, as plateout is dramatically reduced. • A single unit can be used to provide both heat ing and cooling in a single bath . • They are easy to repair. Simple patching or elimination of a single tube can be done depending on the type of heat exchanger with no significant effect on thermal performance. No special welding techniques are required. Most repairs can be handled by plant maintenance people. There is no need to send it out to a special weld ing shop, which can require several weeks. • They are compact and flexible. They can be installed out of the way. and shaped to fit into tank spaces. They are extremely cost effective when the total life and replacement cost of metal units are considered.
Modern Electroplating by F. Louenbeim
801 pages
$180.00
This is the third edition of this valuable textbook. Numerous authorities have contributed summaries on principles of plating and metal finishing, control, maintenance. deposit properties . and troubleshooting. A worthwhile reference. Send Orders to: METALFINISHING, 660 White Plains Rd., Tarrytown. NY 10591-5153 For faster service , caIl (914) 333-2578 or FAXyour order to (914) 333-2570 All book orders must be prepaid. Please mclude 5500 shIpping and handling for delivery of each book via UPS in the U 5.. 510.00 for each book shipped express Canada. and 520 00 for each book sh ipped express to aU other countries .
'0
781
Fluoropolymer heat exchangers are the only polymeric heat exchangers to offer the temperature capabilities and corrosion resistance necessary to heat and cool acid- and caustic-based plating bath s. They offer economical solutions to the problems of maintaining precise temperature control of baths for optimum product quality.
INTRODUCTION Polymeric heat exchanger usage has become Widespread throughout today's industry for heat-transfer applications in highly corrosive situations. The principal reason that polymeric heat exchangers are used instead of conventional metal heat exchangers is that all metals are subject to corrosion in chemical media . Althou gh the corrosion rate of certain metals may be acceptable in the chemical species utilized. they all suffer corrosion. and when aggressive chemical media are used. the rate of corrosion becomes prohibitive to economical operation. There are many types of thermoplastic and other polymeric materials available in heat exchangers (polypropylene and polyethylene have been used for some very low-performance applications), but only a few polymers exhibit the chemical and temperature capabilities that make them suitable for widespread heat-exchanger application. In general, the polymers that exhibit these properties are the fluorocarbon family of resins . They are available from several different resin manufacturers and each exhibits a general range of acceptable usage.
FLUOROPOLYMER STRUCTURE AND PROPERTIeS The wide acceptance of fluoropolymer heat exchangers can primarily be attributed to the unique properties of the fluorocarbon polymers. Fluoropolymers are corrosion res istant to almost all chemicals. with the key to this corrosion lying in the chemical structure. Polytetrafluoroethylene (PTFE). perfluoroalkoxy (PFA). and fluorinated ethylene propylene (FEP) fluoropolymers are fully fluorinated polymers; that is. each branch terminates with a fluorine atom (see box) . Polyvinylidene fluoride (PVDF). ethylene tetrafluorethylene (ETFE). and ethylene-ehlorotrifluoroethylene (ECfFE) are only partially fluorinated (some branches do not end with a fluorine atom) . This fully fluorinated structure provides a polymer that is both chemically inert and thermally stable to high temperarures. Partially fluorinated polymers sacrifice some chemical and thermal resistance 10 enhance their mechanical properties at room temperature. This results in the higher ambient temperature tensile strength and modulus shown in Table I. As can clearly be seen. however. the upper operating limits of the PVDF, ETFE, and ECTFE are severely restricted and FEP and PFA have higher temperature limits. This is important in heating applications when thermal margins of safety can be extremely important. One can also readily see that the fully fluorinated structure of FEP and PFA (fable II) offers superior chemical resistance compared with the other part ially fluorinated polymers. Thi s is especially true in the area of strong alkalies, polar solvents. and strong acids. Two fluoropolymer family members. EFTE and ECfFE. have to date found more applications in structural and lining applications and not in heat-exchanger equipment.
THERMAL PROPERTIES Fluoropolymers are not good heat conductors. Shown in Table nr is a tabulation of thermal conductivity values comparing the various fluoropolymers directly with those of
769
Table I. Properties of Fluorinated Polymers
PTFE
Type
Specific weight Melting point
glee
Tensilestrength
mPa psi mPa psi
2.17 327 621 30 4,350 10 1,450 500
'C
'F
Yield stress Elongation Elasticmodulus
%
mPA psi XIlY
600
87 60 160
Shore D hardness Impact strength (1200)
ft-Ib/in. 2 Heat Distortion Temperature 66 psi (0.46 mPA) Linearexpansion coefficient Thermal conductivity
2.15 270 518 20 2,900 12 1,740 350 500 72 57 200
PFA 2.15 305 581 31 4,500 15 2,250 300 700 101 62
PVDF
ETFE
1.78 178 350 54 7,830 46 6,670
1.75 275 527 24 3,480 24 3,480 500 1,500 217 75
1.7 240 464 31 4,500 31 4,500 300 1,650 240 75
ISO 2,400 348 79
ECTFE
JIm 3 "C
'F
1O-5K
BTU/hr/ft'
4
121 250
ISO
70
73 164
148 300
104 219
115 240
13
13
12
12
12
12
0.20 1.4
0.23 1.6
ft2 -"Ffm Upperservice temperature
FEP
"F
400 205
500 260
"C
0.22 1.5 500 260
0.19 1.3 300
ISO
0.23 1.6 280 140
0.15 1.0 340 170
EcrFE; ethylene-chlorotrifluoroethylene. ETFE, ethylene tetrafluoroethylene: FEP. fluorinated ethylene propylene. PFA. perfluoroalkoxy, PTFE. polytetrafluoroelhylene, PVDF, polyvinyhdene fluonde,
metals. In light of these relatively poor thermal properties, why then does a fluoropolymer work as a practical heat exchanger? When dealing with a heat-transfer situation, one first determines the amount of heat necessary to be transferred. It is then simply a direct function of the heat-transfer coefficient or the capability of the heat exchanger, the area available for heat exchange, and the temperature driving force used to promote that heat transfer.
Q
=
UA~TL
(1)
Here, there is a log mean temperature difference, because in the dynamic situations occurring during tank heatup or cooling, there is a constantly changing system, not a constant temperature. The log mean temperature is defined by the following equation:
~TL
(2)
Note that it is not a simple temperature difference with which one must deal. The main consideration in this article, however, is calculation of the heat-transfer coefficient, as it develops from the relatively low thermal conductivity.
U (3)
770
WHEN YOU'RE DEALING W H CORROSIVE SOLU IONStYOU NEED A HEAT EXCHANGER HAT CAN HANG IN THERE FOR A WHILE. Nothing hangs .1 there like an AMElEK SuperCOli ot Tel'on' . as Tel'on COl1S1nJCtlOO makes n the tJitmate CO!TOSlOn resstaot heat exchange<. unsurpassed f1 rEJSISt"19
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fir1lSht'19 sootoos. They can be "1S1a11od .1tO VY1lkllly any tank configuratIOn. for apl~ocatlOOS
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~
Table II. Chemical Resistance of Fluoropolymer Resins
PTFE
FEP
PFA
PVDF
Media
20t:
6Ot:
9Ot:
20t:
6Ot:
9Ot:
20t:
6Ot:
9Ot:
20t:
6Ot:
9Ot:
H 2S04 (15%) H 2S04 (98%)
A A A A A A A A A A A A A
A A A A A A A A A A A A A
A A A A A A A A A A A A A
A A A A A A A A A A A A A
A A A A A A A A A A A A A
A A A A A A A A A A A A A
A A A A A A A A A A A A A
A A A A A A A A A A A A A
A A A A A A A A A A A A A
A A A A A A A B B A B A A
A A A A A A A B B A U A A
A A A A A A B U U A U B B
aci
(36%)
HNO, (50%) Nitric (20%)/HF (5%)
HF (40%) Chromic (50%) NaOH (50%) NaOCI
0; (wet) Ketones, esters Aromatics Chlorinated solvents
A, acceptable, B, condlllonaIly acceptable, U, unsatisfactory. FEP, fluonnated ethylene propylene; PFA, perfluoroalkoxy, PTFE,polytetrafluorethylene; PVDF, polyvinylidene fluonde.
CALORPLAST: Heat Exchangers That Last! • Thin (2.2") profile uses minimum tank space e
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George RscherCAlORPlAST Heat Exchangers are manufactured of tough impa et-resista nt NDF, polyethyle ne, or polypropylene to provide excellentcorrosion resistance in severe applications. Ideallysuffed for hoofingor cooling most corrosive acids in metal finishing fchrome/tin/
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nickel/ sulfuric anod izing). The modular custom design permils conformityto all)' vessel geo melly. The patented inieclion averrnolding ensures permanent attachment of the tubing to exchanger manifolds. Available in immef1)ion as wellas shell-and-tubetype design.
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Table III. Coefficients of Thermal Conductivity
tr- 'FUn.
Material
Thermal Conductivity W/M"K.
BTU/hr
Copper Graphite Nickel Tantalum Carbon steel Titanium Stainless steel Ni-Cr-Mo PFA
2.700 IBO 619 368 348 III 104 56 \.6 1.5
FEP
1.4
PVDF
\.3
PTFE
390 120 89 50 50 16
15 8 0.23 0.22 0.20 0.19
FEP. fluorinated ethylene propylene; PFA. pernuoeoalkoxy: PTFE. polytetrafluoroethylene; PVDF. polyvinyltdene fluoride.
In this simple definition of the heat-transfer coefficient (Eq. 3), the individual film coefficients are summed up; i.e., what happens to the fluid outside the tube in the bath (ho ) . conduction of heat through the wall (hw). transfer of heat between the fluid in the tubes to the wall (hi)' plus fouling factors. This summation yields the overall coefficient or U value, which is a measure of the ability of a heat exchanger to transfer heat. Taking the example shown here and applying typical data, film coefficients for inside the tubing (condensing steam 500 BTU/hr/ft 2 oP or greater) and in the surrounding bath (200 to 300 for agitated systems) can be inserted. Typical textbook fouling factors are 0.001. If small-diameter tubing is considered, typically \1i-in. diameter tubing with a 0.00125- in. wall (discussion of this typical value is later in the text), the conductivity calculates to a coefficient of 96 BTU/hr/ft 2oF. U
I 500
-
I 96
I 300
+- +-
57.8
+ 0.001 + 0.001
(4)
Plugging all of these values into the equation for the overall heat-transfer coefficient (Bq. 4), it is obvious that the most significant and controlling factor in this calculation is the wall of the tubing. The order of magnitude of this resistance is greater than that in the fluids on either side, and especially so for the fouling factors. For this reason, when sizing any fluoropolymer heat exchanger, fouling factors are not considered to be an important item. Since the order of magnitude is so small, compared with the controlling factor, it becomes mathematically insignificant. The nonstick character of fluoropolymers further enhances the logic that fouling factors are not important. This U value of approximately 60 is typical for steam heating in a plating bath with pure fluoropolymer tubing. The wall conductivity of a metal heat exchanger (based on thermal conductivity is approximately 75 times that of the fluoropolymer heat exchanger) results in an overall heat-transfer coefficient typically three times that of the tluoropolymer heat exchanger. If the mathematics as shown in Equation 5 are reviewed, it can be seen that the order of magnitude of the fouling factors is very significant for a metal heat exchanger, and, therefore, must always be considered when calculating necessary heat-transfer area.
U
= -
I
500
I
I
1500
300
+ -- + -
150
+ 0.001 +0.001
(5)
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Coating and Surface Treatment Systems for Metals -A Comprehensive Guide to Selection by j. Edwa rds 470 p ages
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Selectio n of the most appropriate coating or other surface treatment is addresse d in this comprehe nsive guide , Part r covers 76 industrially important coating types from acrylic polymers through zinc alloys. Part U prov ides an overview of the 19 most important coating treatment methods with emphasis on the implications for a particular product in terms of its substrate or shape . Part III offers a guide to coating characteristics.
Send Ord ers to: METAL FINISHING 660 White Plains Rd., Tarrytown , NY 10591-5153
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Approximately 4 yearsago, a fluoropolymer tubingcalled "Q" tubing was introduced. It effectively doubledthe thermal conductivity over that of all pure fluoropolymers available. If that wall conductivity number is doubledto approximately 200 BTU/hr/ft2°F, the wall is still the controIling factor, but now you have closed the gap considerably with metals (Eq, 6).
FLUOROPOLYMER STRUCTURES AND PROPERTIES PTFE-Polytetrafluoroethylene: MeltingPoint620·P(327·C) F P P F ServiceTemperature SOO°F (260°C) I I I I -C-C-C-CChemically inert Insoluble I I I I
F F F F
FEP·Fluorlnated ethylene propylene:
F F P F
I I I I
-C-C-C-C-
I I I I
MeltingPointS40°F (280°C) ServiceTemperature 400"F(20S·C) Chemically inert Insoluble
F F F CF3 PFA-Perfluoroalkoxy:
F F F F
I I I I
-C-C-C-C-
I I I I
F F F OC3F7 ETFE·Ethylene tetrafluoroethylene: F F H H
I I I I
-C-C-C-C-
I I I I
F F H H PVDF-Polyvlnylldene fluoride: F H F H
I I I I
-C-C-C-C-
I I I I
MeltingPointS80°F (301°C) ServiceTemperature SOO"P (260°C) Chemically inert Insoluble
MeltingPointS12·F(267°C) ServiceTemperature 300"F(I SO·C) Reacts-strong acids/bases Insoluble MeltingPoint320°F(160°C) ServiceTemperature 280°F(I40·C) Reacts-strong acidslbases Solubleinketones
P H P H
ECIFE-Ethylene-chlorotriOuoroethylene: F F H H MeltingPoint46S·F (240·C) I I I I ServiceTemperature 340·P(I70·C) - C- C- C- CGood-acidslbases I I I I Poor-organics & solvents P CI H H
n6
Table IV. Thermal Efficiency versus Tubing Diameter Tubing Diameter
Wall Conductance Coefficiau BTUlhr1tr- m«
(in .;
(mm)
Type
Kca/lhrM~= "C
0.100 0.100 0.125 0.125 0.175 0.175 0.250 0.250 0.3175 0.3175 0.375 0.375
2.54 2.54 3.18 3.18 4.45 4.45 6.35 6.35 8.06 8.06 9.53 9.53
FEP
586 1.170 468 928 336 668 234 468 185 366 156 322
Q
FEP Q
FEP Q
FEP Q
FEP Q
FEP Q
120 240 96
19<1 69 137 48 96 38 75 32 64
F£P. IIUOnllal@ eihylenepropylene; O. a Ituorpolymcrtu6mg
U
I
I
I
-+-+-+0.001 +0.001 500 200 300
84 (6)
The coefficient now typically ranges up to 120 BTU/hr/ft2'F against a typical metal heattransfer coefficient range of the order of 150 to 300 BTU/hr/ft2F. What one must do in a fluoropolymer heat exchanger to match the performance of a metal unit is. therefore. to specify more surface area; however. because of the generally smaller diameter of the tubing utilized in f1uoropolymer heat exchangers. the surface area available per running foot or per cubic foot of heat exchanger is significantly higher than that of the metals; therefore. the unit. while it contains more surface area. does not occupy a higher volume and does not detract from available bath area.
FLUOROPOLYMER HEAT EXCHANGERS Fluoropolymer heat exchangers are generally manufactured with small-diameter tubes. Functionally. it becomes very difficult to gain heat-transfer efficiency in tubing above 'h-in. diameter. The wall thickness required to provide structural strength of tubing larger than that reduces the thermal efficiency considerably. Table IV shows the various sizes of tubing available in units in both FEP and Q. The smaller diameters have significantly higher wall conductivity and, therefore. create a much more thermally efficient unit; however. the smaller diameter tubes are generally suitable only for clean fluids inside the tubes . One generally makes immersion units out of smaller diameter tubes where clean steam or cooling water flows in the tubes. Larger chemical process units are made out of the larger diameter tubes where dirty process fluids are introduced tubeside in the heat exchangers. Units of this latter type are also appropriate for external heaters/coolers for metal finishing baths. Because tubing is extruded from fluoropolymer resins. pressure capability is a strong function of temperature. The pressure capabilities of the various fluoropolymer resins are very much regulated by the temperature of the application as well as the upper temperature capabilities of the various polymers (Fig. 1). Most manufacturers of f1uoropolymer heat exchangers follow a similar pattern. Tubing is typically manufactured with what is called a 10% wall. In other words. the wall thickness is 10% of the diameter of the tubing. Some tubing can be made as low as 8% wall without serious effects on its pressure capability. With the use of the 10% rule for wall thickness. all tubing within this size range (less than 'h in.
m
'C
160
10
·IS
\
140
\
n
]I
149 .. ..177- - - "204
1103
9n
\
821
\ Internal P,..uure 1. Tubl.9
\. \
~
~
~ ~EP
40
~
Eatlrml 'rlssu'" 20
121
0" Tubtll9
~ ~
"PfA
~ i'-. """--
~ ...JA
21S
us
f
fEP
o so
100
ISO
200 2SO MIl .... TDl'EIlATUIlE 'f
JOO
3S0
coo
Fig. 1. Operating limits. diameter) has generally similar temperature/pressure capabilities. Differences in tubing capability created by the hoop stresses in this size range are relatively insignificant and considered to be nonexistent. COMMERCIAL USE This discussion will focus on what can be broadly characterized as the metal finishing industry. Technically, all establishments that apply coatings or surface treatments to a metal item are part of the metal finishing industry. Many additional operations also fit into this general category, such as mechanical polishing, painting, and the application of other coating materials. These areas seldom require heat exchange and, as such, the involvement is limited to the plating and similar chemical or electrochemical finishing operations where corrosive solutions and heating or cooling are involved. Some categorizations of the type of operations included are: • • • •
Decorative plating Hard chromium plating Precious metal plating Chloride , cyanide, fluoride, fluoborate, fluosilicate, pyrophosphate, sulfate, and sulfamate plating solutions for cadmium, chrom ium, copper, gold, lead-tin, nickel , platinum, rhodium, silver, tin, and zinc electroplating • Electroless copper and nickel plating solution s • Deionized water • Anodizing, brightening, clean ing, electrocleaning, electropolishing, etching, neutralizing, passivating, pickling, rinsing, stripping, and surface-activating solutions containing alkalis or acetic, chromic, hydrochloric, hydrofluoric, nitric, oxalic, phosphoric, or sulfuric acids Some of these applications are designed to provide decorative finishes to make the article more attractive . More importantly, they can provide funct ional finishes that impart specific
na
Fig. 2. Minicoi/.
Fig. 3. Supercoi/.
engineering properties such as surface hardness and resistance to wear. abrasion. or corrosion to the article being plated. In all of the above operations, precise control of bath temperature at an optimum condition is critical to obtaining a product of the highest quality. Baths must be heated from room temperature to provide the optimum temperature for processing and then maintained at that condition, either through additional intermittent heating or by cooling. especially in the case of electroplating where the electric energy that is input into the bath is translated into significant heat values. Fluoropolymer heat exchangers can provide heating as well as cooling in a single unit. Electric or flame heating requires a second unit for cooling. In today's metal finishing baths, heating and cooling with fluoropolymer heat exchangers can be accomplished in a number of ways. The first, and perhaps the simplest, technique is an immersion heater/cooler placed directly in the plating bath itself. Several types of these heat exchangers are available. The smallest of these units is a board- or frame-mounted unit available from several vendors (Fig. 2). They are most typically used in small plating baths, but through modular-type construction can provide significant area. They have the deficiency of being fabricated from '.!.I-in. or larger diameter tubing, which provides an inefficiency of operation because of the thicker walls. They also are of a single-tube design, requiring individual tube connections into manifolds for operation with the heating or fluid media. The single-tube design can also limit coolant flow. The second category of immersion coil is that typified by the supercoil (Fig. 3) in sizes ranging from approximately IOto 75 ft2 in heat-transfer area. These are produced in both FEP and Q tubing utilizing Vla- and 'h-in. diameter tubing. These types of coils utilize a process called honeycombing. which fuses the many small-diameter tubing ends into a single fluorocarbon sheath (which is provided with a mechanical connection to a standard NPT Teflon or steel piping connection). In this way. a client can connect the coil utilizing standard supply piping and is not required to manifold many small tubes individually. Larger coils are
779
Fig. 4. S/im/ine. also produced called slimline immersion coils (Fig. 4). These can provide up to 250 ft2 in a single coil and are held in a rigid configuration by external Teflon-covered steel rods. Of course, external tank heating is also a possibility and is utilized by many metal processors. This is simply utilization of a shell and tube unit with fluoropolymer tubes contained in a steel or noncorrosive shell for the heating/cooling media (Fig. 5). Detailed selection of materials can be made for the shell and sealing gaskets, depending upon the nature
Fig. 5. External bath heating.
780
of the media involved. In shell and tube units . it is advisable to use slightly larger diameter tubing , as the bath solution is being circulated through the tubing of the heat exchanger, and particulate contamination as developed in the bath can be circulated through the unit .
ADVANTAGES OF FLUOROPOLYMER HEAT EXCHANGERS Fluoropolymer heat exchangers are true heat exchangers. They are not heaters. They can be used for heating and cooling of corrosive bath environments. Follow ing are some of the advantages compared with some of the competitive heating techniques. • F1uoropolymer heat exchangers have near universal corrosion resistance. This results in long life, no plating bath contamination from corrosive products, and a smaller spares inventory because f1uoropolymers work in all solutions, which provides extreme versatility. You can use the same heat exchangers even though the bath composition is changed. • F1uoropolymer heat exchangers are electrically nonconductive. They are simple. easy to install, and electrically insulated pipe connections are not needed. You get improved plating efficiency because stray currents are not attracted to a nonconductive fluoropolymer heat exchanger. There is no shock hazard. Generally, operating costs are lower than with electric heat . There are no burned-out heaters, which need frequent replacement. • Units are safer to operate as there is no risk of tank ignition or damage to tank lining if solution level drops. This compares not only to an electric heater situation, but also to nonelectric heat sources, such as flame or carbon "hot sticks ." • F1uoropolymers resist fouling and plateout. So, surface passivation may be eliminated in certain solutions. • There is longer bath life. and less bath depletion as compared with direct steam or hot water injection. There is less waste for disposal or recovery. • Original heat transfer performance is maintained. It does not deteriorate with plateout, as plateout is dramatically reduced. • A single unit can be used to provide both heat ing and cooling in a single bath . • They are easy to repair. Simple patching or elimination of a single tube can be done depending on the type of heat exchanger with no significant effect on thermal performance. No special welding techniques are required. Most repairs can be handled by plant maintenance people. There is no need to send it out to a special weld ing shop, which can require several weeks. • They are compact and flexible. They can be installed out of the way. and shaped to fit into tank spaces. They are extremely cost effective when the total life and replacement cost of metal units are considered.
Modern Electroplating by F. Louenbeim
801 pages
$180.00
This is the third edition of this valuable textbook. Numerous authorities have contributed summaries on principles of plating and metal finishing, control, maintenance. deposit properties . and troubleshooting. A worthwhile reference. Send Orders to: METALFINISHING, 660 White Plains Rd., Tarrytown. NY 10591-5153 For faster service , caIl (914) 333-2578 or FAXyour order to (914) 333-2570 All book orders must be prepaid. Please mclude 5500 shIpping and handling for delivery of each book via UPS in the U 5.. 510.00 for each book shipped express Canada. and 520 00 for each book sh ipped express to aU other countries .
'0
781